Electrodeposition of platinum



Patented llrlar. 14, I233 UNITED STATES PATENT/OFFICE ALAN RICHARD POWELL AN'D ARTHUR WILLIAM SCOTT, OF LONDON, ENGLAND, AS-

SIGNORS'TO JOHNSON-MITRE! & COMPANY LIMITED, OF LONDON, ENGLAND mnc'monrzeo srrron or PLATINUM Ho Drawing. Application filed .Tanuary 22, 1932, Serial Ni). 588,235, and in Great Britain Hatch 12, 1931;

This invention relates to the electrolytic deposition of platinum from .aqueous solutions and to the preparation of suitable plating salts for this purpose.

It has already been proposed to deposit platinum electrolytically as a bright plate from baths consisting of a solution of platinum hydroxide (hexahydroxyplatinic acid) in caustic alkalies. This type of bath usually contains about 4 er cent of free alkali metal hydroxide and 1S operated at temperatures below centigrade with a current supplied at 2 volts. Although a bath prepared in this manner may yield good bright deposits for some time after being freshly made'up, it rapidly becomes turbid and a deposit of hydrated platinum trioxide forms on the anode; with more prolonged use a. heavy flocculent. precipitate of various platinum com ounds is formed, whereby the efiective platinum content of the bath is reduced and the deposits'become dull, patchy and loosely adherent.

It has also been proposed to employ neutral solutions containing potassium platmate and magnesium borocitrate as baths for plating purposes, but these baths'have found no commercial application. If these baths are rendered even slightly alkaline a grey tloceulent de sit of metallic platinum is produced.

e present invention has for its principal object to overcome these dificulties and to provide stable baths from which bright platmum deposits may be obtained, of almost any desired thichess. I

According to this invention we utillze an alkali metal platinate bath to which is added a stabilizer and which is then rendered almost neutral by the addition of a. mineral acid. This bath gives no deposit of platinum on the cathode when operated at temperatures below 40 centi rade, but at higher temperatures especially a ve about 65 centigrade a brilliant, white, smooth and firmly adherent deposit is obtained.

As a stabilizer we prefer to use substances which form soluble complexes with platinum,

such, for example, as oxalic, phosphoric or nitrous acids, or salts thereof. We have obtained particularly good results by the use of with water.

an alkali metal oxalate, as'for example sodium oxalate which prevents the formation of platinum trioxide. or compounds thereof on the anode, and retards or prevents the development of'a flocchlent precipitate in the bath, thus rendering it stable over long periods.

In order to obtain good results withthis bath we have found it necessary to keep the free alkalinity corresponding with a sodium hydroxide content of less than 2.5 per cent and advantageously between 0.2 and 1 per cent. As the platinum is removed from the electrolyte by plating, the free alkalinity of the bath will increase and consequently it is necessary to keep this increase within the limits just mentioned by occasional neutraliza- 100 ounces 'of sodium chloroplatin'ate (NagPtClo,6HgO) are dissolved in 6 gallons of hot water and 100 ounces of sodium hydroxide in solid form are then added to the hot solution. The liquid boils vigorously so that cold water must be added as required to keep the boiling under control. When all the sodium hydroxide has been added, the

solution is diluted to 12 gallons with hot water and boiling is continued for from 3 to 4 hours until the-colour of the liquid changes from orange to pale yellow; when the liquid has cooled slightly, a solution of 16 ounces of oxalic acid is added to form 17 ounces of sodium oxalate in the mixture and this is followed by 37 ounces of sulphuric acid diluted The platinum solution will now contain 17 ounces of free sodium hydroxide and after dilution to 24 gallons it contains approximately 1 per cent of latinum and v 0.5 per cent of free sodium hy ro xide. The

sodium chloride, formed by the reaction of the sodium hydroxide with the sodium chloroplatinate and the sodium sulphate, formed by the subsequent partial neutralization with sulphuric acid, act 'as conducting salts and increase the conductivity and throwing power of the plating bath.

Example 2 alkalinity of less than 2.5 per cent and preferably of from 0.2 to 1.0 per cent, therequisite quantity of sodium oxalate, for example 0.5 per cent and suflicient sodium sulphate, for example 3 per cent to give a good throwing power. y In the aforesaid examples any other alkali metal hydroxide maybe usedin place of sodium hydroxide and any other soluble compound of tetravalent platinum, except-cyanide complexes, may beused in place of the sodium hexahydroxyplatinate.

These baths are operated above 40 centirade and preferably at 65 to 85 centigrade with a current density of upto 20 amperes per square foot at 1.5 volts or higher;there is no evolution of hydrogen at the cathode and the current efiiciency is practically 100 i per cent. Bright adherent deposits of platinum are obtained with these baths on polished gold, silver, copper, brass, bronze and nickel-silver surfaces, but nickel, iron and steel surfaces must first be coatedwith copper and/or silver from an alkali metal cyanide I bath.

Satisfactory plates are produced until the platinum content of the bath falls to about. 0.4 per: cent; replenishment of the bath with platinum is then efiected by adding the req- I uisite quantity of platinic hydroxide prepared as hereinbefore described. This. may be added as a wet sludge, but is preferably previously dissolved in the minimum quantity of alkali metalhydroxide solution. In

. the latter case sulphuric acid equivalent to this alkali metal hydroxide must be added to the plating bath after the replenishing solution has been added so as to maintain the al- 'kalinity at approximately constant value.

What weclaim is -1. A process for the electrodeposition of platinum in the form of a bright plate in which the electrolyte comprises a .solution containing an alkali metal platinate, a con- I ducting salt, a stabilizing salt and an. alkali metal hydroxide in an amount equivalent to less than25 percent of free sodium hj- I droxide.

2. In the process as claimed in claim 1 employing as the stabilizing salt an alkali metal oxalate, preferably sodium oxalate.

' 3. In the process as claimed in claim 1 maintaining the free alkalinity at the equivalent of 0.2 to 1 per cent of sodium hydroxide.

4. In the process as claimed in claim 1 0perating the bath at a temperature above 40 centigrade and preferably at to 85 centigrade. 1 a

5. In the process as claimed in claim I replenishing the bath by the addition of a solution of hydrated platinicoxide in an alkaline solution.

6; In the process as claimed in claim 1 controlling iJlN alkalinity Within the limit specified-in claim 1 by the occasional addition of an acid and preferably sulphuric acid.

7*. In the process as claimed in claim 1, controlling the alkalinity at the equivalent of 0.2 to 1% of sodium hydroxide by the occasional adddition ofan acid and preferably sulphuric 301' Y 1- In testimony, we have signed our names to this specification at, London, England, this 4th day of J anuary,- 1932.

ALAN RICHARD POWELL. 

